The invention relates to compressors or pumps, and in particular to hermetic reciprocating piston compressors having a wrist pin assembly in the piston.
A reciprocating compressor typically includes one or more cylinders each adapted to receive a piston for reciprocable compression action. As is commonly known, the piston includes a piston head sealed about the inner annular surface of the cylinder with a connecting rod driving the piston head to provide the necessary reciprocating action. The connecting rod comprises a first and second aperture, the first aperture being connected to the crankshaft and the second aperture being connected through a wrist pin to the piston.
Most piston wrist pins are usually of the full floating type in that the wrist pin is free to rotate both within the connecting rod aperture and the piston bushing. Load applied to the wrist pin during compressor operation is developed from two sources, the forces of inertia from the reciprocating piston or connecting rod and the forces resulting from compression of gas in the cylinder. Wrist pins are generally made of case hardened, high carbon steel to withstand high loads and are accurately ground to needed sizes and tolerances.
Various attempts have been made by the prior art to maintain the wrist pin in a substantially center position within the piston to prevent the wrist pin ends from rubbing against the cylinder wall during piston reciprocation. A typical wrist pin assembly includes circumferential grooves machined in the ends of the piston bore to accommodate wire formed or stamped snap rings known as wrist pin retainers. As disclosed in U.S. Pat. No. 5,111,737, at times a sub-assembly of the wrist pin is necessary before inserting the wrist pin into the piston bore. This patent includes an annular groove machined into the bore into which the retainer is inserted. After assembly, the retainer coacts with the annular groove to secure the wrist pin within the bore. A major problem includes pin bore groove pound out, which is especially common with pistons made from aluminum, hypoeutectic or eutectic alloys Commonly called "pound out" the problem is initiated by an applied reversing load and high frictional wear between the wrist pin and piston.
Another disadvantage of the grooved wrist pin bore and piston are difficulties in machining circular grooves in pistons made from powered metal or aluminum hypoeutectic alloys. Inadequate quality of surface finish and high wear rates of tools are some of the reasons preventing usage of standard tools for machining such materials.
Another known way of maintaining the wrist pin in the proper location is by the use of small disks or buttons stamped from sheet teflon for use as retainers. The buttons are slightly circumferentially compressed and inserted in the bore on either side of the wrist pin. The disadvantage with this design is that under the effects of gravity and applied forces, the wrist pin and buttons sometimes move downward. This causes the teflon retainer button, at its lowest position, to be in continuous contact with the cylinder wall during reciprocating movement of the piston. Such sliding contact between the cylinder wall and button surface will increase energy loses due to the additional friction. The friction of the sliding teflon button, which is approximately 0,020-0.040 inches thick, against the cylinder wall, will additionally gradually wear out the button. The wrist pin then will finally be in rubbing contact with the cylinder wall while teflon particles will penetrate the oil in the bearing surfaces and valve system of the compressor.
The present invention is directed to overcoming the aforementioned problems associated with prior wrist pin assemblies.